1. Field of the Invention
The present invention relates to a method of measuring a cable delay time.
2. Description of the Related Arts
In data processing systems for high-speed processing, a central processing unit (CPU) in particular, is constituted by high-density semiconductor IC elements. Signals between these elements are weak, and coaxial cables are used to prevent noise mixing.
The delay time of the signal transmission through the connection cables greatly influences the processing times or phases of various signals, and since cables having the same length may have different delay times, the delay times of the respective cables must be measured.
A demand has arisen for a method of easily and accurately measuring the delay times of connection cables when the connection cables are prepared.
One conventional method of measuring a cable delay time is practiced as follows. One end of a cable to be measured is connected to a pulse genertor, probes of a conventional sampling scope (referred to hereinafter as an oscilloscope) are connected respectively to both ends of the cable, and a time difference between the pulse waves from the probes is directly read on the screen of the oscilloscope.
Another conventional method of measuring a cable delay time is practiced as follows. One end of a cable to be measured is connected to a pulse generator, and the other end of the cable is kept open. The probes are connected to the pulse generator to read a time difference between the transmitted waveform of the pulse signal and the waveform reflected by the cable end, and half of that difference is measured to obtain the cable delay time.
According to these conventional methods, the two waveforms on the screen of the oscilloscope must be read with reference to the scale thereon, and large reading errors tend to occur. In particular, if the delay time is increased, the measuring range must be changed, thus further increasing the possibility of errors.
According to the first method described above, the cable must be measured after a characteristic impedance is connected to theend of the cable, and it is cumbersome to connect the characteristic impedance element to or disconnect it from the cable for every measurement.